if (parentThread != null) {
            try {
                parentThread.join(millis);
            } catch (final InterruptedException e) {
                logger.warn("Interrupted job at {}", parentThread.getName());
            }
        }
    }

    /**
     * Cleans up the crawled data for a given session.
     * Deletes URL queue data and access result data, and clears the URL filter.